[Question] Understanding of access hazard & ABA hazard about cs431 HOT 1 CLOSED

ABA hazard vs Access hazard

As you said, ABA hazard happens when memory locations pointed by a particular pointer changes from A -> B -> A but the first A and second A may have different origin (thus we should view it as a different object). Such situation happens if one thread changes the pointer from A -> B and deallocates A, but another thread reallocates A by chance and changes the pointer from B -> A. It is considered as a severe problem since we cannot distinguish the first A and the second A by simply reading from the pointer and comparing two addresses. So the essence of ABA problem is misjudgment by naive comparing memory accesses.

On the other hand, access hazard only considers invalid memory accesses (such as use-after-free). Thus it describes different problem with ABA hazard.

Line 5

Line 5 merely accesses .next field of t, but it does not make any judgement by making a comparison. As a result, line 5 is subject to only access hazard, but not ABA hazard.

Line 6

Line 6 is indeed ABA hazard since even if the latest value of tail is equal to t, the latest value of tail may have different origin than t (consider a situation where t is deallocated by another thread and reallocated) but we are making judgement that tail has not been changed so far by such comparison.

from cs431.